1. Field of the Invention:
The invention is directed to a modulatable, separately excited high frequency generator for a sinusoidal power output.
Description of the Related Art
Such a circuit for a power amplifier operating in the so-called F-mode is, for instance, known from the book by Kovacs: "High Frequency Applications of Semiconductor Devices"; Franzis Publisher Munich, 1977, page 387. The known circuit comprises a MOS-FET-transistor (metal oxide semiconductor field effect transistor) operating in the switching mode, which is matched with a .lambda./4-transformer. A parallel resonant circuit at the output tuned to the fundamental frequency short-circuits the harmonic waves. Because of the .lambda./4-transformer, there develops at the drain point of the field effect transistor, a short-circuit behavior for even harmonics and an idling behavior for odd harmonics.
Because of the idling behiavior, the drain current does not contain any odd harmonics of the fundamental frequency. The even harmonics of the fundamental frequency, on the other hand, circulate in short-circuit and therefore do not consume any power. This results in a very high efficiency of the circuit, which, in the ideal case, amounts to 100%. In the actual application case, the efficiency of the circuit is reduced by the relationship between the impedance R of the .lambda./4 line and the saturation resistance R.sub.ON of the switching transistor according to the formula .eta..sub.F =R/(R+2 R.sub.ON). The efficiency is thus greater by approximately 27% than that of the class B push-pull power amplifier which is routinely used these days.
That this known circuit principle has up to today found no practical entry into the technology, should be traceable to several reasons. One reason is to be seen in the necessity of having to utilize a .lambda./4 line; such a line has a length of several meters in the technical frequency range. Another reason would lie in the lack of sufficiently powerful switching transistors, in particular, field effect transistors. A third cause can be found in that a modulation of the power output is only possible by means of a direct voltage supply, whereby, with the usual technology, the overall efficiency of the circuit again drops below that of a class B power amplifier.